Currently, liquid crystal elements are components most extensively used in flat panel displays which are space-saving man-machine interfaces. Particularly, a liquid crystal element of a so-called active matrix type (e.g., TFT system), in which an active element (switching element) such as a transistor is arranged for each of pixels, becomes the mainstream of the flat display panel.
Recently, on the other hand, attention is being given to self-luminous devices provided for flat panels. The self-luminous devices include plasma light-emitting elements, field emission elements, electroluminescence elements, and so on.
Among them, the electroluminescence elements (hereinafter, referred to as “EL elements”) are broadly divided into inorganic EL elements and organic EL elements. The inorganic EL element is an AC-driven thin-film EL element using an inorganic semiconductor, and an inorganic material such as ZnS is mainly used therein.
Regarding the organic EL element, there is an example in old times, in which light is emitted by applying voltage onto an anthracene-deposited film (Thin Solid Films, 94 (1982) 171), or the like. However, an opportunity of extensively attracting attention as a light-emitting device was that C. W. Tang et al. demonstrated that high-luminance light emission can be achieved with DC drive using a laminated structure of thin films of a diamine-based molecule and a fluorescence metal chelate complex. In recent years, applied researches for making the organic EL element into a device as a light-emitting element that affords a high-speed response and a high efficiency, including material developments have been vigorously conducted in terms of advantages of easiness in making a large area, desired coloring which is obtainable by the developments of various kinds of new materials, capability of being driven at low voltage, and so on as compared with the inorganic EL element.
The organic EL element is a carrier-injection self-luminous device utilizing luminescence generated when an electron and a hole which arrive at a light-emitting layer are recombined together. In FIGS. 1A to 1C, a cross-sectional configuration of a typical organic EL element is schematically illustrated. In FIG. 1A, reference numeral 11 denotes a metal electrode, 12 denotes a light-emitting layer, 13 denotes a hole-transporting layer, 14 denotes a transparent electrode, and 15 denotes a transparent substrate. In FIG. 1B, reference numeral 16 denotes an electron-transporting layer. In FIG. 1C, reference numeral 17 denotes an exciton dispersion barrier layer. In FIGS. 1B and 1C, furthermore, the same reference numerals as those of FIG. 1A represent the same structural components, respectively.
As the light-emitting layer 12 of FIG. 1A, an aluminum-quinolinol complex having electron-transporting properties and light-emitting properties, typically Alq3 represented by the following formula (1) or the like, is used. In addition, as the hole-transporting layer 13, an electron-donative material such as a triphenyl diamine derivative, typically α-NPD represented by formula (1) or the like, is used. Furthermore, as shown in FIG. 1B, an organic compound layer composed of three layers of the electron-transporting layer 16, the light-emitting layer 12, and the hole-transporting layer 13 is often used.

Furthermore, the light-emitting layer may be made of a single material. In many cases, however, pigment doping by which a pigment material having a high light emission efficiency is doped in a host material is often used.
In the configurations of FIGS. 1A to 1C, the metal electrode 11 is used as a cathode and the transparent electrode 14 is used as an anode for taking out emitted light, and the organic compound layer is sandwiched between both electrodes. In general, each layer of the organic compound layer has a film thickness of about several tens of nm. As a metal material of the cathode, a metal having a small work function, such as aluminum, aluminum-lithium alloy, or magnesium-silver alloy. In addition, a conductive material having a large work function is used as an anode, such as indium tin oxide (ITO).
The organic EL element is capable of emitting three primary colors of red, green, and blue, and so on in a self-luminous manner by appropriately selecting materials that constitute the light-emitting layer, so that it is possible to constitute a full-color display device. In addition, it has excellent characteristics of a high-speed response and a wide-viewing angle with respect to a liquid crystal display, so that it is expected as a next-generation flat panel.
There are two representative methods as a process for realizing a full-color display device using an organic EL element.
One of them is a patterning method through a vacuum vapor deposition method using a shadow mask, which is considered in a monomer material and the other of them is a patterning method through an inkjet method which is considered in a polymer material.
In the monomer material, a method of obtaining an organic thin film using a vacuum vapor deposition method is a technique most popularly used. However, for realizing a RGB full-color panel with high definition patterning, a process for filling with different colors by means of a shadow mask is a process having a high difficulty even though it is not difficult to fill roughly divided areas of several kinds of light-emitting layers with different colors. On the other hand, in the case of the inkjet method in the polymer material, it is difficult to keep the uniformity of the polymer thin films, so that it is said that it still needs considerable time to realize a practical RGB full-color display device.
Under such circumstances, attention is focused on white-light-emitting organic EL element. The white-light-emitting organic EL element has a much broader range of applications, such as a white light source, interior illumination, a flat backlight source for a liquid crystal display, and a monochrome display. A combination of a color filter technology, which achieves an actual accomplishment in the liquid crystal display and a white-light-emitting organic EL element, simply realizes a full-color display device at low cost without filling the RGB-light-emitting layers with different colors in a complicated manner as described above.
Presently, a pigment material by which white light emission having sufficient characteristics can be obtained has not been realized by a single light-emitting material. Therefore, for realizing a white-light-emitting organic EL element, there is a need of mixing three primary colors of RGB, or blue and a complementary color of yellow, so that various systems have been considered. Among them, as a method of mixing three colors of RGB, the following two types can be considered:    (1) a single light-emitting layer type, where each pigment of RGB is doped into a single light-emitting layer; and    (2) a RGB laminated layer type, where light-emitting layers of RBG are laminated. Regarding (1), there are documents, such as Applied Physics Letter (Appl. phys. Lett. vol 67, 2281 (1995)). Regarding (2), there are documents, such as Science (Science vol 1267, 1332 (1995)). The single light-emitting layer type is the simple one as the light-emitting layer can be formed of a single layer. In the case of the RGB laminated layer type, it is comparatively easy to realize the optimization with the doping concentration in each layer and each film thickness.
As described above, the present development of organic EL element has progressed in a wide range. Considering wide-ranging applications, how to increase the light emission efficiency becomes important. For increasing the efficiency of the organic EL element, extensive attention has been focused on a phosphorescent (triplet) light-emitting material in recent years.
In the organic EL element, holes and electrons injected from the electrode come to excitation states by recombining with each other in the light-emitting layer (hereinafter, this kind of chemical species is referred to as exciton). The light is emitted in the process of causing transition to base state. In this process, there are a singlet excitation state and a triplet excitation state in the excitation states, and the transition from the former to the base state is referred to as fluorescence, and the transition from the latter is referred to as phosphorescence. The substances under these states are referred to as singlet exciton and triplet exciton, respectively.
In most of the organic EL elements which have been studied until now, fluorescence at the time of transition from the single exciton to the base state is used. In recent years, on the other hand, elements that actively utilize phosphorescence emission through the triplet exciton have been studied.
Representative documents that have been published are as follows.
Document 1: Improved energy transfer in electrophosphorescent device (D. F. O'Brien et al., Applied Physics Letters Vol 74, No 3 p 422(1999))
Document 2: Very high-efficiency green organic light-emitting elements based on electrophosphorescence (M. A. Baldo et al., Applied Physics Letters Vol 75, No 1 p 4(1999))
In these documents, there is mainly used a configuration in which four layers are laminated as the organic compound layer which is sandwiched between electrodes. The materials to be used are a carrier transporting material represented by the above formula (1) and a phosphorescence light-emitting material.
An abbreviation of each material in the formula (1) is as follows.    Alq3: Aluminum-quinolinol complex,    α-NPD: N4,N4′-Di-naphthalen-1-yl-N4, N4′-diphenyl-biphenyl-4,4′-diamine,    CBP: 4,4′-N,N′-dicarbazole-biphenyl,    BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,    PtOEP: Platinum-octaethyl porphyrin complex, and    Ir(ppy)3: Iridium-phenyl pyrimidine complex.
In each of Documents 1 and 2, an element that obtained a high efficiency was of the configuration of FIG. 1C and was an element prepared by dispersing and mixing about 6% of platinum-octaethyl porphyrin complex (PtOEP) or iridium-phenyl pyrimidine complex (Ir(ppy)3), which is a phosphorescent light-emitting material, in host materials of α-NPD as the hole-transporting layer 13, Alq3 as the electron-transporting layer 16, BCP as the exciton dispersion barrier layer 17, and CBP as the light-emitting layer 12.
The organic light-emitting element using a phosphorescent light-emitting material can be principally expected to be become high efficient for the following reasons. Excitons generated by a carrier recombination of holes and electrons include singlet excitons and triplet excitons with a ratio of 1:3. Fluorescent light-emission has been used in the conventional organic light-emitting elements, and the upper limit of the yields of light emission thereof was 25% in principle with respect to the number of excitons being generated. However, using phosphorescence generated from the triplet exciton, at least triple yield can be expected in principle. Furthermore, considering it together with transition from singlet excitation to triplet excitation which is energetically higher than the singlet excitation by means of inter system crossing, it is expected that the yield of light emission is 100% which is 4 times higher in principle.
The documents in which the light emission from the triplet excitation state is described include Japanese Patent Application Laid-Open No. 11-329739 (organic EL element and method for manufacturing the same), Japanese Patent Application Laid-Open No. 11-256148 (light-emitting material and organic EL element using the same), Japanese Patent Application Laid-Open No. 8-319482 (organic electroluminescent element), and so on.
As described above, the phosphorescent light-emitting materials have the possibilities of largely improving the efficiencies of the conventional organic EL elements. The same is applicable to the white-light-emitting EL element, and it is also considered that the phosphorescent light-emitting material is a promising material for increasing the efficiency of the white-light-emitting EL, so that the material is expected to realize a new white light source having luminous efficacy similar to that of fluorescent lamp.
However, when an organic light-emitting element for white light emission with a high efficiency is prepared using the phosphorescent light-emitting material as described above, a light emission color is shifted to red beyond expectation. Since it was used as a white light source, sufficient color purity could not be obtained or the like in some cases. This is not limited to the organic light-emitting element for white light emission. In the organic light-emitting element using a phosphorescent light-emitting material, a desired color purity could not be assured in some cases even though it is designed such that the light emission color becomes a specific color while selecting materials to be mixed.